Glen Canyon Dam Adaptive Management Program Administrative History

It is apparent that before emplacement of the dam gully degradation in terraces was restored by periodic alluvial deposition from river floods, but perhaps even more important is the redistribution of flood sands onto higher terraces by wind. Thus, we propose the term "restorative base-level hypothesis" to emphasize the dynamic equilibrium between gully erosion and renewed deposition, a process that remains active in Cataract Canyon but is disrupted in Grand Canyon by the presence and operation of the dam.

We developed type geomorphic settings to develop a conceptual process model for the diverse small-catchment geomorphic system in Grand Canyon. Research findings explain how streams are able to cross broad, flat terraces given a rainfall event and how they become progressively more integrated with the river. The primary channelization processes are ponding and overflow, alluvial fan progradation, and infiltration and piping, all of which contribute to nickpoint migration. An understanding of these processes was essential to building the geomorphic model.

The predictive mathematical model quantifies erosional vulnerability by applying a hypothetical rainfall event of 25 mm/hour onto a catchment above a "pristine" terrace sequence. The principal driving factor for erosion is basin area. The principal resisting factor for erosion is terrace diffusion capacity, which is a function of terrace sand cross-sectional area and infiltration capacity. Several important modifying factors are applied to the basic model to determine relative vulnerability of each terrace to gully erosion. Vulnerability of the top terrace at each catchment is plotted against the measured amount of gully erosion in that terrace, providing a base line against which progressive changes in gully depth can be easily monitored in the future.

Field studies and research show that: (1) gully erosion of terraces has been severe during the past 20 years in Grand Canyon due to unusually high precipitation; and (2) sediment deprivation coupled with the lack of large annual floods has caused a reduction in restorative (depositional) factors. Continued measurement and documentation of geomorphic processes in catchments, particularly at type geomorphic settings, will further refine and verify the predictability of the model. We conclude that beach-habitat-building flows are essential for initiating natural restorative processes and that one of the most important processes in gully mitigation may be eolian reworking of newly deposited flood sands onto higher terraces. Prior to the construction of Glen Canyon Dam, gully-deepening and river/wind depositional processes were in dynamic equilibrium, allowing the preservation of ancient cultural sites for the past several thousand years.

An ecological survey of the riparian zone of the Colorado River from Lees Ferry to the Grand Wash Cliffs, Arizona, was initiated between 1 June 1974 and 30 June 1976. The purposes of this study were:

First, to describe vegetational changes as a result of the controlled water release from Glen Canyon Dam, second, preparation of a vegetation map from river level up to the 500 foot contour level, third, to describe population densities, home ranges, and demography of important vertebrates, fourth, to inventory insects of the riparian zone, fifth, to describe the distribution and impact caused by feral burros, and sixth, to describe the interrelationships of humans with the biota.

The major findings include the following: (1)The construction of Glen Canyon Dam has permitted the development of a new riparian community. This community is characterized by salt cedar, arrowweed, coyote willow, desert broom, and seep willow. (2) Botanical investigations in the riparian and adjacent habitats discerned the presence of 807 species of vascular plants representing 92 families. Also, two species, previously undescribed, Flaveria mcdougallii and Euphorbia rossii, are presented. (3) An accessment of important vertebrates and insects revealed: a) rodent communities on beaches tend to be less productive and less stable than those rodent communities of the terrace areas, b) Peromyscus eremicus appears to be the most successful small mammal in the riparian zone, c) rodent survivorship is very low and suggests a nearly annual population turnover, d) 178 species of birds utilize the riparian zone, of these 41 breed there, e) the most common bird species is the Lucy's Warbler, f) over 12,000 insect specimens in 20 orders and 247 families were collected and prepared, g) insect production on the exotic salt cedar fluctuate dramatically in comparison to insect production on dominant native plants. (4) Feral ass distribution was found to be greater than previously believed. It has been determined that the expanding feral ass populations are systematically destroying riparian and desert habitats within the study area and their immediate removal is suggested. (5) Human impact seems to be a function of visitor activities and the specific biotic sensitivity of the use area rather than a function of the total number of users. (6) In 1974, 395 different campsites were reported between Lees Ferry and Pierce's Ferry. In 1975, 350 different campsites were used. (7) Establishment and maintenance of an inner canyon trail system, the removal of all future human fecal waste material and education of river users may be the means to minimize habitat destruction rather than just setting a user-day limit.